Why have we been arguing, friend? We're seeing much the same thing here. 100% agree Sandy Leon is replacement level. He has 2319 career minor league PA with an OPS of .654. He has 1023 MLB PA with an OPS of .656. In 12 professional seasons, he's had an OPS over .700 twice. 2016 was a miracle half season for him, providing 2.5 of his career 2.6 fWAR. It's safe to say he's a .650ish OPS hitter. His solid defense makes him a serviceable MLB backup.
Vazquez was billed as a generational defender whose bat would lag behind. But he was decent at the plate through the minors as an age-advanced catcher. Full season OPS up until his debut: .665, .863, .713, .766. He's got a career minor league OPS of .734. That doesn't scream "this guy can't and won't ever hit." Regardless of the defense, which has taken a step back, he's been a mixed bag at the big league level. There's a big difference between the .735 guy he was last year (36% of his total MLB PA fwiw) and the .617, .585, and .548 in the other years. This season is the worst season of his pro career. It's really hard to gauge what he is with such a drastic step back this year. I think the expectation of a .675-.725 OPS is realistic, but it's certainly not a given. With good-to-great defense (also not a given, it appears), that can be a 2-win starting catcher.
I've given my pitch on Swihart, which is really that we don't know yet. He, too, had some success coming up through the minors at an early age, followed by a really good age 23 debut season. The fact that he had so much room to grow, both offensively and defensively, is what's enticing. And then he got derailed. But if he can be a legitimate MLB catcher defensively, and it looks that way to me, then it's up to his bat whether he's better overall than Vazquez.
I also agree the Sox have (rightly, imo) hitched their wagon to Vazquez with his extension, and Leon is not going to supplant him. Swihart is the wild card, and, like you say, the choice is/will be between him and Leon. If Swihart can catch, I think it's a no-brainer. I would have even been fine with them moving on from Leon this spring. They obviously weren't confident in Swihart as the backup catcher at that point, though, and the current injuries make that look like a good decision. But Leon isn't really much more than Dan Butler or any other replacement level catcher. I hope to see more of Vazquez and Swihart (and less of Leon) as they come back and in 2019. It remains to be seem whether they are the long term answer(s) at catcher, but it's clear that Leon is not. However, no matter how good or bad they are the rest of this year, I think going into 2019 with Vazquez and Swihart as the only catchers is a lot of risk to take on. I think they'll both be fine going forward, but do you want to have to live with what I think?